Filtration devices comprising a hollow plunger which slides within a hollow tubular vial are frequently employed in laboratory and other environments to remove solids from a liquid sample. For example, biological samples such a blood, mucus or urine may be filtered (to remove contaminants or unwanted protein for example), the filtrate being subsequently used to perform medical or other tests.
In such devices, the hollow plunger typically has an aperture at one end, which is fitted with a filter membrane. The liquid sample to be filtered is initially held in the tubular vial, which is open at one end and closed at the other. The hollow plunger is inserted into the tubular vial, fitting tightly therein, so that a sealing contact is formed between the exterior of the hollow plunger and the interior of the tubular vial. The hollow plunger is then depressed into the tubular vial, with the sealing contact forcing the liquid sample through the filter membrane and into the interior of the hollow plunger. The filtrate is then held inside the plunger, until it is required for subsequent processing. The plunger may be fitted with a cap to prevent the filtrate escaping. When the filtrate is required, the cap may be pierced using a syringe or other device in order to extract the filtrate. U.S. Pat. No. 4,800,020 describes an example of a filtration device of this type.
Hollow plungers and tubular vials of this kind are typically made from, or contain parts made from, a plastics material. Plastics materials are convenient for this purpose because they are relatively cheap and can easily be formed (for example, by injection moulding) with the required dimensions to a relatively high degree of accuracy and with relatively complicated surface features, where required. Furthermore, plastics materials are generally relatively flexible, and therefore resistant to breakage, for example on insertion of the hollow plunger into the tubular vial.
However, plastics materials are often susceptible to leaching by the liquid sample, either prior to or subsequent to filtration, resulting in the liquid sample being contaminated with impurities from the plastics material. This is especially problematic when the liquid sample remains in contact with the plastics material for a prolonged period of time. In particular, it is often desirable or necessary to store the sample for several hours or days (e.g. 72 hours) after filtration, whilst the sample is transported to and processed at a laboratory for example, during which time the liquid sample remains in constant contact with the interior of the hollow plunger, resulting in significant contamination of the liquid sample.
It is an object of the present invention to at least mitigate some of the problems of the prior art.